1. Field of the Invention
The invention relates to a new process for the production of an aluminum hydroxide [Al(OH).sub.3 ] having rounded grain surfaces as well as flameproof plastic materials containing Al(OH).sub.3 as filler.
2. Background Art
Aluminum hydroxide is a filler long known for flameproofing polymeric materials. The Al(OH).sub.3 obtained from the Bayer process is mainly used as a filler either directly or optionally after a grinding process. However, the grain surfaces of these hydroxides are very irregular, sharp-edged and fissured and, as a result, produce a rough surface in the correspondingly filled plastic molded articles.
In the incorporation of ground Bayer Al(OH).sub.3 into plastic, unforeseeable viscosity variations were also observed that raise substantial problems in the manufacturing process. To counteract these difficulties, efforts have been made to coat the surface of the Al(OH).sub.3 particles with silanes (German Patent No. 2,743,682). Besides the additional effort and the additional costs that result from the separate coatings, the processing properties of the so-treated Al(OH).sub.3 are still unsatisfactory.
According to European Published Patent Application No. 011,667, it was then attempted to convert the Al(OH).sub.3 resulting from the Bayer process which is present as agglomerates of individual crystals into a heated aluminum subsaturated Bayer alkaline solution and to stir it for 1 to 25 hours, whereupon the solid is separated. The original agglomerates are separated by this treatment at their grain boundaries. Thus, the individual primary crystals undergo a rounding; moreover, the resulting rounded Al(OH).sub.3 particles have practically no fine-grain portion. This Al(OH).sub.3 shows a very good viscosity behavior in plastic, and moreover the resulting surfaces of the filled plastic molded articles are smooth. However, a substantial drawback in this process is that contaminants enclosed in the original Bayer Al(OH).sub.3 are precipitated onto the resulting Al(OH).sub.3 which leads to discoloration when using unsaturated polyester resins (UP resins).
It was proposed according to European Published Patent Application No. 407,595 that Al(OH).sub.3 aggregates derived from the Bayer process be physically deagglomerated, e.g., by centrifugation or by grinding in a ball mill. But damages to the grain surfaces occur through the physical treatment which has a detrimental effect on the processing properties in the plastic. Moreover, such a treatment involves considerable expense.